In the modern communications network space, signal reach and spectral density are important factors in overall network cost. Assuming other factors to be equal, increases in either signal reach or spectral density tend to reduce overall network cost and are thus very attractive to network service providers.
Signal reach is the distance that an optical signal can be transmitted through a fiber, before conversion to electronic form is required to perform signal regeneration. Using suitable optical amplifiers and optical processing techniques, between 10 and 20 fiber spans (of 40–80 km each) can be traversed by an optical signal before optical/electrical conversion and regeneration are required.
Spectral density, which is normally expressed in terms of bits/sec/Hz (b/s/Hz), is a measure of the extent to which the theoretical maximum bandwidth capacity of an optical channel is utilized. This value is generally determined by dividing the line rate (in bits/sec.) of a channel by the optical frequency (in Hz) of that channel. A spectral density of 1 indicates that, for a given channel, the line rate and optical frequency are equal, so that each carries a bit of information. Existing telecommunications systems commonly operate at line rates of approximately 2.5 Gb/s to 40 Gb/s. At a line rate of 10 Gb/s, current Wavelength Division Multiplexed (WDM) (or Dense Wave Division Multiplexed (DWDM)) transmission systems achieve a spectral density of approximately 0.1 b/s/Hz. If the line rate is increased to 40 Gb/s, the spectral density increases to approximately 0.4 b/s/Hz, illustrating the advantages of increasing the line rate.
However, there is a trade-off involved in using increased line rates to improve spectral density. In particular, increased line rates typically result in a reduction in signal reach. For example, with the use of appropriate optical amplifiers, a signal reach of 2500 km has been demonstrated at a line rate of 2.488 Gb/s (equivalent to a SONET/SDH OC-48 signal). At a line rate of 40 Gb/s, the signal reach drops to approximately 1000 Km. This reduction in signal reach is explained by the fact that, as the line rate increases non-linear optical effects (e.g. self-phase modulation, optical dispersion, etc.) become progressively more significant, and cause increased bit error rates. In general, in order to keep the bit error rate below a tolerable threshold the distance that an optical signal is transmitted through a fiber before conversion to electronic form must be reduced.
Typically, the performance of optical fibers and amplifiers is non-linear across an optical spectrum of interest. This non-linear performance is manifested in, among other things, variations in the signal-to-noise ratio and signal gain (Q) of each channel. These variations tend to accumulate with each amplification stage and thus can become very large over the 15–20 spans of a fiber link. Since the length of each fiber link (i.e. the length of each span, and the number of spans before conversion to electronic form) is governed by the bit error rate of the lowest-performing channel, such high Q variations mean that most of the (higher performing) channels must operate with a shorter signal reach than would be indicated by their individual bit error rates. Accordingly, it is desirable to equalize performance across the channels.
Various methods of optical gain equalization are known in the art. See, for example, U.S. Pat. No. 6,091,538 issued to Takeda et al. on Jul. 18, 2000 and entitled Gain Equalizing Apparatus; and U.S. Pat. No. 6,097,535 issued to Terahara on Aug. 1, 2000 and entitled Method for Optical Amplification and System for Carrying Out the Method. Both of these patents use a variation of known Adaptive Optical Equalization techniques, in which the detected signal power is used to dynamically adjust the gain of an optical amplifier, to thereby minimize variations in the gain across multiple channels. Other known techniques involve the use of static or dynamic gain equalization filters, which operate by attenuating the optical signal power on channels with relatively high gain.
While each of the above methods are capable of reducing variations in gain, the physical properties of installed equalization devices are subject to a certain amount of variation, resulting in an unavoidable equalization error, typically on the order of approximately +0.2 dB. Over a link comprising 20 spans, this optical equalization error can accumulate to produce an uncompensated variation across the channels of as much as ±4 dB.
Accordingly, a method and apparatus that enables maximized signal reach by providing effective equalization across multiple channels remains highly desirable.